1. Field of the Invention
The present invention relates to a separator for fuel cell, a process for production thereof, and a solid polymer type fuel cell using the separator. More particularly, the present invention relates to a separator for fuel cell having a strength necessary for separator even when made in a small thickness, a process for production thereof, and a solid polymer type fuel cell using the separator.
2. Description of the Prior Art
Fuel cells are advantageous in that they require substantially no fossil fuel (whose depletion in the near future is feared) and, when used for power generation, generate substantially no noise and are high in energy recovery as compared with other methods for power generation. Therefore, fuel cells are being developed as a power generation plant of relatively small size for buildings and factories.
Among fuel cells, a solid polymer type fuel cell operates at lower temperatures than other types of fuel cell do; therefore, it has such characteristics that not only the parts constituting the cell are little corroded, but also the cell can discharge a relatively large electric current for the low temperature operation. Therefore, the solid polymer type fuel cell is drawing attention as a substitute electric source for internal combustion engine of automobile.
Currently, a smaller size or thickness is required for various products and parts. In the solid polymer type fuel cell as well, a small volume and a compact shape are required when it is used as a substitute electric source for internal combustion engine of automobile.
Among the parts constituting the solid polymer type fuel cell, the separator is generally a flat plate having a plurality of parallel grooves formed at one or both sides, and is used for transferring the electricity generated inside the fuel cell, to outside.
Some of the conventional fuel cell separators are produced by impregnating a high-density graphite or a graphite material with a thermosetting resin and forming, at one or both sides of the resulting material, a plurality of parallel grooves by machining.
In JP-B-1-57466 is proposed a process for producing a separator by subjecting a mixture of a particular thermosetting resin and a graphite powder to pressure molding.
In JP-A-4-214072 is proposed a process for producing a fuel cell separator by molding a mixture of a phenolic resin, a milled carbon fiber and a graphite powder into a sheet shape and graphitizing the sheet material.
As a fuel cell of small volume and compact shape has come to be required as mentioned previously, the separator of fuel cell (which occupies a large portion of fuel cell) inevitably needs to be formable in a small size or in a small thickness and, when formed in a small thickness, needs to have a strength required for separator, that is, the separator needs to have a high strength and high toughness. However, the conventional separator obtained by subjecting graphite to machining and the separator obtained by subjecting, to pressure molding, a mixture of a particular thermosetting resin (specifically, a very special phenolic resin having a p-xylene bond in the molecular chain) and a graphite powder have had problems in that these separators, when made in a small thickness, are fragile, easily break at the time of fuel cell assembling, and have low reliability.
The fuel cell separator of small thickness obtained by subjecting a mixture of a resin, a fiber and a graphite powder to graphitization, has had problems in that (1) since the graphitization need be conducted by firing a sheet-shaped material at a high temperature of 1,500 to 3,000° C., the resulting separator is expensive owing to the use of a special equipment and method for such graphitization, (2) the sheet-shaped material contracts during the graphitization, resulting in low dimensional reproducibility, and (3) the strain caused by the above contraction remains in the graphitization product.